1. Field of the Invention
The invention relates to immobilization methods and, in particular, for immobilizing indicator molecules on supports such as optical sensors, and to sensors having those indicator molecules immobilized on their surfaces.
2. Description of the Related Art
U.S. Pat. No. 5,356,417 describes a fluorescent-based sensing device comprising indicator molecules and a photosensitive element, e.g., a photodetector. Broadly speaking, in the context of the field of the invention, indicator molecules are molecules where one or more optical characteristics of which is or are affected by the local presence of an analyte. Indicator molecules have been used to measure a wide array of analytes, such as glucose, oxygen, toxins, pharmaceuticals or other drugs, hormones, and other metabolic analytes. These indicator molecules are often classified based on the chemical principal on which their activity is based. For example, the activity of many indicators, such as the one described in U.S. Pat. No. 6,344,360, the disclosure of which is incorporated herein by reference, are based on the principle of luminescence, in particular fluorescence. U.S. Pat. No. 6,344,360 describes a fluorescent indicator molecule containing a fluorescent lanthanide metal chelate complex whose fluorescence emission intensity is increased by the presence of certain sugars. Other categories of indicator molecules include calorimetric indicator molecules which change their color in the presence of an analyte and indicator molecules which change absorbency at a particular wavelength.
Indicator molecules vary widely in their chemical compositions and properties. For example, indicator molecules may be monomeric or polymeric and/or hydrophilic or hydrophobic.
Indicator molecules have been immobilized on supports, such as sensor surfaces, by attaching the indicator molecule at the surface of the support. Immobilizing indicator molecules on inert polymeric substrates poses a particular challenge, since those substrates have to be modified to create or chosen to provide attachment points for the indicator molecules.
Molecules can be immobilized on a substrate in different ways. Immobilization is often based on covalent links between the substrate and the immobilized molecule. To facilitate immobilization, inert substrates are generally avoided, as those require pretreatments, e.g., with U.V. or with harsh acids such as nitric acid to achieve oxidation of the substrate to create reactive sites. These pretreatments are generally associated with undesirables, such as instability of the immobilization product or the handling of harsh acids. Thus, more readily modifiable supports, such as polyacrylamide supports, are generally chosen to attach molecules of interest. A polyacrylamide support can, for example, be readily modified, for example, by attaching through reactive amine groups, which enables attachment of a wide variety of molecules, including haptenes, peptides, carbohydrates and oligonucleotides.
The immobilization of molecules, such as indicator molecules, on optical sensors poses some unique challenges. For example, it is important that the immobilization method does not interfere with the function of the optical sensor. Also, certain immobilization methods might result in the discoloration of a support and thus, interfere with its function as optical waveguide. Known immobilization methods can also change the optical properties of the surface by, e.g., causing bubbling or rippling of the surface, thus disrupting the optical path of a sensor or adversely affect material properties. Thus, there is a continuing need for improvements in the way indicator molecules are immobilized on an immobilization substrate. This need is particularly conspicuous in the context of immobilizing indicator molecules on sensor bodies, in particular on optical sensor bodies, used as long term implants.